A carbon fiber-reinforced composite material (hereinafter also referred to as CFRP (carbon fiber-reinforced plastics)) has an extremely high elastic modulus and strength and is lightweight, and is therefore specifically noted as a metal-substitute material. Accelerated demand for CFRP especially for use for structure materials for an automobile, wind power blades, pressure vessels and aerospace systems is expected, and demand for carbon fibers as well as matrix resins such as epoxy resins for use for CFRP is increasing these days.
In use for structure materials for an automobile, wind power blades, pressure vessels and aerospace systems, CFRP molding methods differ, and therefore the necessary properties of matrix resins for CFRP also differ in such different uses.
For example, wind power blades have become molded through infusion molding, Va-RTM (vacuum assisted resin transfer molding) or Light-RTM. According to these methods, for example, reinforcing fibers are previously arranged in a mold composed of an upper die and a lower die using a film and FRP, and the mold is vacuumed so that an epoxy resin composition to be a matrix resin is filled therein and infiltrated into the reinforcing resins, and then the epoxy resin is cured and molded.
According to infusion molding, Va-RTM or Light-RTM, in general, it takes several tens of minutes before an epoxy resin composition prepared by mixing an epoxy resin and an epoxy resin curing agent could be filled in a mold owing to the characteristic of the molding method. Consequently, the epoxy resin composition to be used in the molding method is required to have a low viscosity and a long pot life. As the epoxy resin curing agent, an isophoronediamine, polyether skeleton-having polyamine compound or the like is used.
For CFRP for pressure vessels, molding according to a filament winding method is employed. A filament winding method is a method where reinforcing fiber yarns infiltrated with a matrix resin such as an epoxy resin composition are used to cover the outer surface of a liner and the matrix resin is cured. Regarding the epoxy resin composition to be used in this method, when the pot life thereof is short and the composition cures rapidly, the epoxy resin may cure in the stage before molding. Accordingly, a rapid-curing epoxy resin composition cannot be used in the filament winding method.
As opposed to this, CFRP for structure materials for an automobile is molded according to high-cycle RTM. This is an improvement of a conventional RTM method.
The conventional RTM method is a type of closed molding that uses a mold formed of a pair of upper and lower dies, in which a fiber-reinforced preform is arranged in the mold, then the mold is sealed up by clamping, and thereafter a resin such as an epoxy resin composition is injected into the mold via an injection hole so as to be infiltrated into the fiber-reinforced preform, and then the resin is cured and released from the mold. However, according to the conventional RTM method, the molding times takes a few hours (for preform arrangement, resin infiltration, resin curing and mold releasing), and therefore for production of CFRP for structure materials for an automobile, a high-cycle RTM method having higher productivity is employed.
Regarding the molding technology according to the high-cycle RTM method, the fiber-reinforced preform arrangement time, the resin infiltration time, the resin curing time and the mold releasing time are greatly shortened, and therefore the total molding time is shortened to 10 minutes or so. In the step from resin infiltration to curing in the high-cycle RTM method, for example, in a high-pressure RTM method that is a type of high-cycle RTM method, reinforcing fibers are arranged in a mold composed of a pair of upper and lower dies and sealed up therein, and the mold is depressurized. Next, an epoxy resin that is the main component of an epoxy resin composition, and an epoxy resin curing agent are pressed into the mixing head in a mist state from separate tanks, and immediately after collision mixing in the head, the resultant mixture is rapidly injected into the mold and infiltrated into the carbon fibers, and the epoxy resin is then cured. After collision mixing, the epoxy resin composition is injected into the mold under high pressure through plural injection nozzles so as to increase the filling speed in the mold and increase the infiltration speed into the carbon fibers.
In the high-cycle RTM method, an epoxy resin and an epoxy resin curing agent are mixed and then immediately injected into the mold, and therefore the pot life of the epoxy resin composition that is a mixture of the epoxy resin and the epoxy resin curing agent is not so much required to be long. On the other hand, however, from the viewpoint of productivity, the infiltration performance into reinforcing fibers and the filling speed into a mold are required to be high, and additionally the curability is required to be high, and therefore, the epoxy resin composition for use in the high-cycle RTM method is required to have a low viscosity and rapid curability.
It is known to use a polyamine compound as an epoxy resin curing gent, and PTLs 1 to 3 disclose an epoxy resin curing agent containing bis(aminomethyl)cyclohexane as a polyamine compound.